The electric grinder is a handheld polishing tool powered by electricity. The technician can use the electric grinder to polish or grind the surfaces of wooden or metallic workpieces.
A Taiwan patent No. M428013 disclosed an electric grinder, which comprises a housing, a brushless motor, a circuit board, a press-button switch, and a varistor. The housing has a first end and a second end. The first end has a head, and the second end has a handle. The handle has a press bar extending toward the first end. The brushless motor is disposed inside the head. The circuit board is disposed inside the handle and electrically connected with the brushless motor. The circuit board has a control panel disposed on the surface of the handle. The press-button switch has a press button, a limit seat and an elastic element. The press button is disposed in the handle and has a connection section extending toward the interior of the housing. The limit seat is fixed to the circuit board. The elastic element is disposed between the press button and the limit seat. The varistor is disposed in the circuit board, and one end of the varistor has an adjust lever joined with the connection section. While using the electric grinder, the user operates the press button to actuate the adjust lever to vary the resistance of the varistor, whereby the circuit board can output different powers to the brushless motor to control the rotation speed of the electric grinder.
Via reviewing the prior-art technology and the structure of ordinary electric grinders, it is found: as the brushless motor is normally a brushless DC motor, phase failure may make the electric grinder unable to operate; further, the cogging torque of the brushless DC motor is unfavorable to frequent start-stop operations and phase inversion of the electric grinder.
Therefore, how to develop an electric grinder adapted to phase failure, frequent start-stop operations and phase inversion has become a problem the manufacturers are eager to overcome.